Electro-pneumatic converters, such as current to pressure transducers are in common use as field instruments mounted in pipeline systems for controlling the process fluid. Accordingly, these devices are installed in potentially hazardous explosive environments as well as a variety of temperature conditions. Such devices generally receive a variable electrical input signal (i.e. a variable current input signal of between 4-20 mA or a variable voltage input signal of between 1-5 V) and eventually provide a variable pressure output to an actuator for a fluid control valve or other similar control device.
There are presently existing current to pressure transducers which employ various temperature compensation circuits which typically use the temperature effect on certain circuit components to counteract the temperature effects on other circuit components. While this form of temperature compensation may be sufficient in a limited number of circumstances, it is not sufficient in the majority of applications because the compensation required is a complex function of the temperature. Accordingly, it has been desired to provide temperature compensation of all functions of an electro-pneumatic device, such as a current to pressure transducer or positioner.
In addition, such electro-pneumatic devices require calibration which entails making manual mechanical adjustments to linkages or potentiometers or some combination thereof, typically on the "bench", before installation of the units, to attain a degree of static accuracy. Normally, for instance, there is a linearly moving valve actuator arm and a non-linearly moving position sensing device (a rotating potentiometer, for instance) which are joined by a positioner feedback linkage. When mounting these units in the field, any slight misalignment between the benchset and the field mounting will reduce the linearity and therefore the accuracy of the device in operation.
Furthermore, to initially calibrate or to recalibrate such electro-pneumatic devices, it is required that covers and protective elements of the device be removed to allow access to the adjusting components. This can be inconvenient, particularly in the case of explosion proof installations that require a major effort to obtain permission from the plant supervisor so that the explosion proof seals and joints can be violated while making the necessary adjustments. Another potentially damaging aspect is that every time the device needs recalibrating, the internal components will be exposed for a period of time to the plant environment in which the device is located. This can lead to degradation of the components and eventual reduction in the reliability of the device.
Accordingly, it is desired to provide an improved microprocessor based electro-pneumatic device which can be adapted for calibration purposes. It is further desired to enable remote calibration of the microprocessor based electro-pneumatic device so that no mechanical adjustments or intrusions to the inside of the instrument need to be made for recalibration. It is also desired to provide an easier more flexible method of initially calibrating the device or to accommodate a wider range of process control applications to which the device might be utilized.
Another desire is to provide an improved microprocessor based electro-pneumatic device which can be used to compensate for potential non-linearities introduced when the device is being field mounted.
It is another desire to provide an electro-pneumatic device which may be calibrated with greater accuracy and wherein no inaccuracy is added to the linearization process such as presently occurs in prior art linearization procedures